1. Field of the Invention
This invention relates to a bar code symbol reading apparatus for reading bar code symbols, and particularly to an apparatus of this type having a double-reading preventing function. The word "double-reading" means an erroneous operation of reading (i.e., decoding) the content of a bar code symbol, then transferring the read data to a host computer or the like, thereafter again scanning and decoding the bar code symbol and transferring the decoded data to the host computer.
2. Description of the Related Art
With the spread of Point of Sales systems (POS), bar code symbols have been found everywhere. There are several tens of types of code systems, which include JAN/UPC/EAN code, ITF (Interleaved 2 of 5) code, CODE 39, NW-7 code, CODE 128, etc., as typical systems. These code symbols are referred to as one-dimensional bar code symbols, and each of them has a data capacity of about ten and several characters.
POS systems are broadly employed in department stores, supermarkets, etc., and bar code symbol reading apparatuses are used as the terminals of the POS systems. It is required to produce a bar code symbol reading apparatus which is cheap, easy to handle, and can perform accurate reading.
The bar code symbol is read basically in the following manner: A light beam is radiated from a light source onto the symbol to thereby scan the same, and then a light beam reflected therefrom is converged onto a light receiving element in the reading apparatus. Subsequently, the output signal of the light receiving element is decoded in an electric circuit comprising a microcomputer, etc., to thereby read the bar code symbol.
There are two types of bar code symbol reading apparatuses--a type of scanning symbols at all times and a type of scanning symbols when a reading switch or the like is operated. In the always scanning type, a "double-reading preventing" function for preventing twice reading of a single bar code symbol is considered especially important. In particular, registers used in supermarkets, etc., must have such a function. If an article with a bar code symbol is kept on a bar code reading apparatus even after the symbol is read, or if the article is removed from the apparatus too slowly, or if the article is moved on the apparatus here and there to read the symbol, a single bar code symbol may be read several times, and thus the number of articles which are bought actually may not correspond to that number of articles which is recorded in the host computer.
To avoid this, in the case of the conventional one-dimensional bar code symbol reading apparatus, two types of double-reading preventing methods have been proposed--one method for judging that a single bar code symbol has been read twice, when the decoded contents of two bar code symbols which have been read successively are identical to each other, and the latter symbol is canceled; the other method for inhibiting reading of the next bar code symbol until a predetermined time period elapses after the content of one symbol is transferred to the host computer.
Recently, the data capacity of the one-dimensional bar code system has been considered too small, and several types of code systems having a large data capacity, which are called "two-dimensional bar code systems", have been developed. One of them, called "data code", is a two-dimensional white/black square pattern, and specifically consists of an L-shaped outer frame, white and black outer frames arranged alternate, and data regions within the outer frames. However, a method for effectively preventing double-reading of such a two-dimensional bar code symbol has not yet been proposed.
If the aforementioned double-reading preventing method suitable for the one-dimensional bar code symbol reading apparatus is applied to the two-dimensional reading apparatus, lots of time is required to compare two-dimensional bar code symbols with each other since they have a data capacity much larger than one-dimensional bar code symbols. Further, decoding two-dimensional bar code symbols is very complicated, and therefore requires much time. Accordingly, the conventional method of comparing decoded data items is not suitable for the two-dimensional bar code symbol reading apparatus.
In addition, in the case of the method for inhibiting reading of the next bar code symbol until a predetermined time period elapses after one symbol is transferred to the host computer, it is very difficult to determine the predetermined time period. If the time period is long, it takes a long time until the next bar code symbol is read, whereas if it is short, a single bar code symbol may be read twice. Moreover, "double-reading" will be performed when the operator has left a sheet with a bar code symbol on the reading apparatus.